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\(\frac{sin^2a+1}{2.cos^2a}+\frac{1+cos^2a}{2.sin^2a}+1=\frac{tan^2a}{2}+\frac{1}{2cos^2a}+\frac{cot^2a}{2}+\frac{1}{2sin^2a}+1\)
\(=\frac{1}{2}\left(tan^2a+1+tan^2a+cot^2a+1+cot^2a+2\right)\)
\(=\frac{1}{2}\left(2tan^2a+4+2cot^2a\right)=tan^2a+2+cot^2a=\left(tana+cota\right)^2\)
B.
\(\frac{1-4sin^2a.cos^2a}{4sin^2a.cos^2a}=\frac{\frac{1}{cos^4a}-\frac{4sin^2a}{cos^2a}}{\frac{4sin^2a}{cos^2a}}=\frac{\left(\frac{1}{cos^2a}\right)^2-4tan^2a}{4tan^2a}=\frac{\left(1+tan^2a\right)^2-4tan^2a}{4tan^2a}\)
\(=\frac{tan^4a-2tan^2a+1}{4tan^2a}\)
C.
\(\frac{sina+tana}{tana}=\frac{sina}{tana}+1=1+sina.\frac{cosa}{sina}=1+cosa\)
D.
\(tana+\frac{cosa}{1+sina}=\frac{sina}{cosa}+\frac{cosa\left(1-sina\right)}{1-sin^2a}=\frac{sina.cosa}{cos^2a}+\frac{cosa-cosa.sina}{cos^2a}\)
\(=\frac{sina.cosa+cosa-sina.cosa}{cos^2a}=\frac{cosa}{cos^2a}=\frac{1}{cosa}\)
Câu C sai
Lời giải:
a)
\(\cos 2a=\frac{2}{5}\Rightarrow \sin ^22a=1-(\cos 2a)^2=1-(\frac{2}{5})^2=\frac{21}{25}\)
Vì $a\in (0; \frac{\pi}{4})\Rightarrow 2a\in (0; \frac{\pi}{2})$
$\Rightarrow \sin 2a>0\Rightarrow \sin 2a=\frac{\sqrt{21}}{5}$
$\tan 2a=\frac{\sin 2a}{\cos 2a}=\frac{\sqrt{21}}{5.\frac{2}{5}}=\frac{\sqrt{21}}{2}$
$\cot 2a=\frac{1}{\tan 2a}=\frac{2}{\sqrt{21}}$
-------------------------
$\sin 2a=\frac{24}{25}\Rightarrow \cos ^22a=1-(\sin 2a)^2=\frac{49}{625}$
$a\in [\frac{-3}{4}\pi; \frac{-\pi}{2}]\Rightarrow 2a\in [\frac{-3}{2}\pi ; -\pi]\Rightarrow \cos 2a< 0$
$\Rightarrow \cos 2a=\frac{-7}{25}$
$\Rightarrow \tan 2a=\frac{\sin 2a}{\cos 2a}=\frac{24}{25.\frac{-7}{25}}=\frac{-24}{7}$
$\Rightarrow \cot 2a=\frac{-7}{24}$
Bài 1:
\(A=\left(1+sinx\right)\left(1-sinx\right)tan^2x=\left(1-sin^2x\right).\frac{sin^2x}{cos^2x}=cos^2x.\frac{sin^2x}{cos^2x}=cos^2x\)
\(B=cot^2x-sin^2x.cot^2x+1-cot^2x=1-sin^2x.\frac{cos^2x}{sin^2x}=1-cos^2x=sin^2x\)
\(C=tan^2x+2+\frac{1}{tan^2x}-\left(tan^2x-2+\frac{1}{tan^2x}\right)=2+2=4\)
Bài 2:
Đề yêu cầu tính giá trị lượng giác nào bạn? sin?cos?tan?cot?
Không hỏi thì làm sao mà biết cần tính gì
\(A=\frac{tana+tanb}{tan\left(a+b\right)}-\frac{tana-tanb}{tan\left(a-b\right)}=\frac{tana+tanb}{\frac{tana+tanb}{1-tana\cdot tanb}}-\frac{tana-tanb}{\frac{tana-tanb}{1+tana\cdot tanb}}\\ \Leftrightarrow A=1-tana\cdot tanb-1-tana\cdot tanb=-2tana\cdot tanb\)
\(B=\frac{cos^3x-cos3x}{cosx}+\frac{sin^3x+sin3x}{sinx}\\ B=\frac{cos^3x-4cos^3x+3cosx}{cosx}+\frac{sin^3x+3sinx-4sin^3x}{sinx}\\ B=\frac{-3cos^3x+3cosx}{cosx}+\frac{-3sin^3x+3sinx}{sinx}\\ B=\frac{cosx\left(-3cos^2x+3\right)}{cosx}+\frac{sinx\left(-3sin^2x+3\right)}{sinx}\\ B=-3cos^2x+3-3sin^2x+3=6-3\left(sin^2x+cos^2x\right)=6-3=3\)
~~~~~~~~chúc bạn học tốt~~~~~~~~~~
\(A=\frac{\frac{sina}{cos^3a}-\frac{cosa}{cos^3a}}{tan^3a+3+\frac{2sina}{cos^3a}}=\frac{tana.\frac{1}{cos^2a}-\frac{1}{cos^2a}}{tan^3a+3+2tana.\frac{1}{cos^2a}}\)
\(=\frac{tana\left(1+tan^2a\right)-\left(1+tan^2a\right)}{tan^3a+3+2tana\left(1+tan^2a\right)}=\frac{3\left(1+9\right)-\left(1+9\right)}{27+3+2.3.\left(1+9\right)}=...\)
\(a,\left(\frac{tan^2x-1}{2tanx}\right)^2-\frac{1}{4sin^2x.cos^2x}=-1\)
\(VT=\left(\frac{tan^2x-1}{2tanx}\right)^2-\frac{1}{4.sin^2x.cos^2x}=\left(\frac{1}{tan2x}\right)^2-\frac{1}{sin^22x}=\left(\frac{cos2x}{sin2x}\right)^2-\frac{1}{sin^22x}=\frac{cos^22x-1}{sin^22x}=\frac{-sin^22x}{sin^22x}=-1=VP\)
b, \(VT=\frac{cos^2x-sin^2x}{sin^4x+cos^4x-sin^2x}=\frac{cos2x}{\left(sin^2x+cos^2x\right)^2-sin^2x-2.sin^2x.cos^2x}=\frac{cos2x}{1-sin^2x-2.sin^2x.cos^2x}=\frac{cos2x}{cos^2x-2.sin^2x.cos^2x}\)
=\(\frac{cos2x}{cos^2x.\left(1-2.sin^2x\right)}=\frac{cos2x}{cos^2x.cos2x}=\frac{1}{cos^2x}=1+tan^2x=VP\)
d, \(VT=\left(\frac{cosx}{1+sinx}+tanx\right).\left(\frac{sinx}{1+cosx}+cotx\right)=\left(\frac{cosx}{1+sinx}+\frac{sinx}{cosx}\right).\left(\frac{sinx}{1+cosx}+\frac{cosx}{sinx}\right)\)
\(=\left(\frac{cos^2x+sinx.\left(1+sinx\right)}{cosx.\left(1+sinx\right)}\right).\left(\frac{sin^2x+cosx.\left(1+cosx\right)}{sinx.\left(1+cosx\right)}\right)=\left(\frac{cos^2x+sinx+sin^2x}{cosx.\left(1+sinx\right)}\right).\left(\frac{sin^2x+cosx+cos^2x}{sinx.\left(1+cosx\right)}\right)\)
=\(\frac{1}{cosx.sinx}=VP\)
e, \(VT=cos^2x.\left(cos^2x+2sin^2x+sin^2x.tan^2x\right)=cos^2x.\left(1+sin^2x.\left(1+tan^2x\right)\right)=cos^2x.\left(1+tan^2x\right)=cos^2x.\frac{1}{cos^2x}=1=VP\)
c, \(VT=\frac{sin^2x}{cosx.\left(1+tanx\right)}-\frac{cos^2x}{sinx.\left(1+cosx\right)}=\frac{sin^3x.\left(1+cosx\right)-cos^3x.\left(1+tanx\right)}{sinx.cosx.\left(1+tanx\right).\left(1+cosx\right)}\)
=\(\frac{sin^3x+sin^3x.cotx-cos^3x-cos^3.tanx}{\left(sinx+cosx\right)^2}=\frac{sin^3x+sin^2xcosx-cos^3x-cos^2sinx}{\left(sinx+cosx\right)^2}=\frac{sin^2x.\left(sinx+cosx\right)-cos^2x.\left(sinx+cosx\right)}{\left(sinx+cosx\right)^2}\)
\(=\frac{\left(sin^2x-cos^2x\right).\left(sinx+cosx\right)}{\left(sinx+cosx\right)^2}=\frac{\left(sinx-cosx\right).\left(sinx+cosx\right).\left(sinx+cosx\right)}{\left(sinx+cosx\right)^2}=sinx-cosx=VP\)
Đây nha bạn
đề bài đầy đủ: rút gọn các biểu thức lượng giác sau trên điều kiện xác định của chúng:
\(\frac{sin^2x}{cosx+cosx.\frac{sinx}{cosx}}-\frac{cos^2x}{sinx+sinx.\frac{cosx}{sinx}}=\frac{sin^2x}{sinx+cosx}-\frac{cos^2x}{sinx+cosx}=\frac{sin^2x-cos^2x}{sinx+cosx}\)
\(=\frac{\left(sinx+cosx\right)\left(sinx-cosx\right)}{sinx+cosx}=sinx-cosx\)
\(\left(\frac{sinx}{cosx}+\frac{cosx}{1+sinx}\right)\left(\frac{cosx}{sinx}+\frac{sinx}{1+cosx}\right)=\left(\frac{sinx+sin^2x+cos^2x}{cosx\left(1+sinx\right)}\right)\left(\frac{cosx+cos^2x+sin^2x}{sinx\left(1+cosx\right)}\right)\)
\(=\left(\frac{sinx+1}{cosx\left(1+sinx\right)}\right)\left(\frac{cosx+1}{sinx\left(1+cosx\right)}\right)=\frac{1}{sinx.cosx}\)